1. Field of the Invention
The invention disclosed herein relates to the field of gas turbines. In particular, the invention is used to provide control of turbine blade tip clearance.
2. Description of the Related Art
A gas turbine includes many parts, each of which may expand or contract as operational conditions change. A turbine interacts with hot gases emitted from a combustion chamber to turn a shaft. The shaft is generally coupled to a compressor and, in some embodiments, a device for receiving energy such as an electric generator. The turbine is generally adjacent to the combustion chamber. The turbine uses blades, sometimes referred to as “buckets,” for using energy of the hot gases to turn the shaft.
The turbine blades rotate within a shroud ring. As the hot gases impinge on the turbine blades, the shaft is turned. The shroud ring is used to prevent the hot gases from escaping around the turbine blades and, therefore, not turning the shaft.
The distance between the end of one turbine blade and the shroud ring is referred to as “clearance.” As the clearance increases, efficiency of the turbine decreases as hot gases escape through the clearance. Therefore, an amount of clearance can affect the overall efficiency of the gas turbine.
If the amount of clearance is too small, then thermal properties of the turbine blades, the shroud ring, and other components can cause the turbine blades to rub the shroud ring. When the turbine blades rub the shroud ring, damage to the turbine blades, the shroud ring and the turbine may occur. It is important, therefore, to maintain a minimal clearance during a variety of operational conditions.
Therefore, what are needed are techniques to reduce clearance between turbine blades and a shroud ring in a gas turbine. The techniques should be useful for a variety of operational conditions.